Calling bpy.ops.text.save() from '3D_VIEW' operator execute method
About 'INVOKE_DEFAULT'  and context override
While you can often override a context not all operators support it.
Examples of context overrides, in particular this one: context override in text editor, see also a more extensive explanation of context overriding. Avoid doing this whenever possible, there is in this case an alternative.
A more sane alternative.
I recommend that you spawn your own file save dialog - a template for that code exists. See Text Editor &gt; Templates &gt; Python &gt; Operator File Export
To save a Text Datablock to disk is easy.
destination = '/home/zeffii/Desktop/file.txt'

text_reference = bpy.data.texts['Text']
with open(destination, 'w') as outfile:
    outfile.write(text_reference.as_string())